1. Field of the Invention
The present invention relates generally to gyroscopic reference systems of the type including a plurality of strapped down, force-rebalanced gyroscopic rate sensors for measuring craft rotation rates about primary craft axes, together with a system for computing from such measures aircraft stabilization and attitude data. A typical strapped down system is disclosed in U.S. Pat. No. 4,212,443 entitled "Strapped down Attitude and Heading Reference System for Aircraft Employing Skewed Axis Two Degree of Freedom Rate Gyros" and assigned to Sperry Corporation. More specifically, the invention related to improved torque feedback control for two degree of freedom rate sensors.
2. Description of the Prior Art
Strapped down gyroscopic inertial reference apparatus for aircraft and space vehicles is well known to those skilled in the art of gyroscopic aircraft control systems; many rate gyro configurations and control systems based thereon have been described extensively in the literature. In general, such systems include a plurality of rate sensors strapped down to the vehicle for measuring the angular velocity of the vehicle about its primary axes, which measure, along with vehicle acceleration and heading measures, is supplied to a digital computer to provide output data for use in stabilization, control, navigation, or guidance of the aircraft. Since the gyros are strapped to the airframe the rate sensors are preferably of the force or torque rebalancing type; that is, the gyro is maintained substantially aligned with its support case by feeding the gyro pick-off signal back to the gyro torquer in a manner to maintain the pick-off signal value essentially null, the torquer current so required being a measure of the rate being sensed by the rate gyro. Typical two-degree-of-freedom rate sensors of this type are disclosed in the T. R. Quermann U.S. Pat. No. 3,529,477 for a "Gyroscopic Rotor Suspension" issued Sept. 22, 1970 and in the C. G. Buckley, J. A. Kiedrowsky U.S. Pat. No. 4,189,948, issued Feb. 26, 1980 for a "Permanent Magnet Torquer for Free Rotor Flexure Suspended Gyroscopes", both inventions being assigned to Sperry Corporation. It will be appreciated that the ultimate output of the gyroscopic rate sensor must be compatible with the requirements of available digital computer techniques.
An apparatus for making the output of the gyroscopic rate sensor compatible with digital computers is disclosed in U.S. Pat. No. 4,222,270, by Arnold R. Allen, entitled "Gyroscope Rate Range Switching and Control System", issued Sept. 16, 1980, and assigned to Sperry Corporation. The Allen patent discloses a pulse width modulation type of analog-to-digital conversion apparatus useful in strapped down inertial gyroscope reference systems which utilize plural force-balanced gyro rate sensors for sensing aircraft body rates about principal craft axes wherein gyro torquer feedback currents are maintained precisely proportional to sensed craft body rates and a precision digital count proportional thereto is desired. The invention incorporates a digital counting technique for providing a precision digital count proportional to the current supplied to the torquer. Such a count is generated by using a high frequency clock to quantize a lower frequency rectangular wave that has been pulse width modulated in proportion to the amplitude of the corresponding gyroscope error signal, the rectangular wave determining the magnitude of the zeroing feedback currents supplied to the gyro torquer coil. The one-way digital counter is synchronously enabled by the same rectangular wave and because it uses the same high frequency clock to provide a counter clocking frequency, that counter output, compared with a reference count, is the desired digital number corresponding to the currents supplied to the gyro torquer and hence corresponding to the sensed rate. The precision currents supplied to the gyro torquer are controlled by a VMOS power switching transistor circuit. Adverse effects of any transients associated with the rise and fall of the pulse width modulated rectangular wave currents, particularly at the positive and negative extremes, are eliminated by the use of pairs of guard band pulses at the beginning and end of the modulator duty cycle that are used predictably to control the rise and fall of the torquer currents and to blank the counter, thereby eliminating any count uncertainties at the switching times and assuring that erratic torquing transient current is not supplied to the torquer.
As described in detail in the referenced Allen patent, the torquer input signal is a series of 500 Hz current pulses having modulated pulse widths which vary from half positive and half negative to substantially full positive and full negative (within the 95% and 5% duty cycle due to the guard band pulses) in accordance with the gyro pick-off signal. It should be noted, however, that there is undesirable transformer-like coupling between the torquer coils and the pick-off coils which has a deleterious effect in producing a net torque to be applied to the gyro rotor. This net torque results from an erroneous pick-off signal being nulled and the action of the rotor suspension spring constants and auto-ejection (windage, etc.) phenomena which results in a torque being applied to the gyro that is not asked for and hence producing an undesired drift to the gyro which varies with rate inputs. It should be further noted that because of the close spacing between the torquer coils, if the trailing edges of the pulse width modulator rectangular waves of each channel of the gyro fell substantially at the same time, or very close to the same time, the changing magnetic fields of one pick-off coil would transformer couple into the coil of the other thereby producing a distorted pulse width waveform and an undesired bias drift in the presence of input rates. Accordingly, there is a need for an apparatus which minimizes the transformer-like coupling which occurs in flexure suspended gyroscopes of the type having pulse width modulated torquer signals.